Wave movements in oceans and large lakes is an important source of energy that can be utilized by harvesting energy from the waves by means of wave power assemblies, also called wave energy converters, which are positioned or anchored at locations with suitable wave conditions.
A number of different types of wave energy converters for harvesting and converting wave energy into electrical energy are previously known. So-called linear generators, which, by means of a rope or other connecting means, can convert vertical movements of a buoyant body caused by wave movements into a reciprocating movement of a generator coil or rotor of a linear generator anchored to the sea or lake floor, are one example. The reciprocating movement of the generator coil/rotor generates in its turn electrical current in the windings of an adjacent stator of the linear generator anchored to the floor.
Another previously known type of wave energy converter comprises a wave energy absorbing buoyant body with an energy absorption and conversion system, which may be placed on the sea floor. The buoyant body is connected to a winch by a winch wire. The winch and the winch wire connect the buoyant body to a reference body below the water surface, such as an anchor platform under the water surface, an anchor on the sea floor, or another anchoring device. As the wave forces causes the buoyant body to move in the longitudinal direction of the winch wire, the winch is forced to rotate, whereupon the rotating motion of the winch axle produced can be converted to electricity by means of an energy conversion system.
Still another type of previously known wave energy converter is instead based on relative movement between, on the one hand, a buoyant body and a thereto attached so-called acceleration tube, and, on the other hand, a working piston reciprocable in acceleration tube, wherein the relative movement is caused by wave movements in the body of water where the wave energy converter is anchored by means of one or several mooring lines to harvest wave energy. The movement of the working piston can be used for driving for example a pump unit, such as a double-acting hydraulic pump or a hose pump, a hydraulic motor and/or a hydraulic turbine of an energy conversion system, which is disposed within or adjacent to the buoyant body for producing electricity which can be transmitted to an energy storage or electrical grid.
When harvesting electric power from wave energy, it is desirable to be able to place several wave energy converters in an array in the vicinity of each other in a location with particularly favourable wave conditions to achieve an efficient harvesting of wave energy. By arranging for example ten, twenty, or even more wave energy converters in such an array, also called a wave power station, several advantages are achieved. One advantage is that one and the same service vessel can carry out check-ups and routine maintenance on all wave energy converters in the wave power station without unnecessary transportation, which saves time, personnel and costs. Another advantage is that the electric power generated by the wave energy converters in a wave power station can be transmitted to shore via a common cable, which decreases the transmission losses and the risk of cable failures, and reduces the cost for laying power cables.
At least one connection substation enabling electrical connection of input power cables from individual wave energy converters to a common output power cable is required to be able to transmit electric power generated by the wave energy converters in a wave power station to shore.
In wave power stations where the energy conversion system of the individual wave energy converters is placed on an ocean or lake floor surface, or on a submarine structure anchored to the floor surface, which is usually the case with e.g. linear generators, it is most natural to also arrange the connection substation or stations on or at the floor surface, i.e. under the water surface. Such positioning of an electrical connection substation under the water surface, however, puts very high demands on waterproofing of the station and may also lead to high costs for underwater equipment and divers, for example when repairs and connections or disconnections of individual wave energy converters are to be carried out.
In wave power stations where instead the individual wave energy converters are of a type where the energy conversion system is placed adjacent to a buoy or buoyant body floating on or near water surface, as is usually the case e.g. with wave energy converters with acceleration tubes, it would be possible to eliminate or considerably reduce the costs for underwater work if also the connection substation or stations is/are designed to float on or near the water surface.
A floating connection substation for wave energy converters is previously known from the patent publication SE 507 925, which discloses a power plant consisting of 10 separate buoy power stations with turbines of a so-called Savonius type. The buoy power stations feed a collecting station in a separate central buoy via power cables. The central buoy is provided with suitable equipment, such as breakers and disconnectors, transformer and protective relays. From the central buoy, a common cable is run to shore via the sea floor. The individual buoy power stations are anchored, preferably with three anchor chains displaced 120° in relation to each other so that the buoys cannot rotate with the turbine. According to what is disclosed in SE 507 925, the central buoy can also be anchored in a similar manner, but otherwise no further details are given as to how the central buoy/connection substation is designed and constructed.
After an extensive period of inventive work combined with practical tests, the present inventors have realized that such a floating connection substation, which in itself is advantageous, has to be designed in a thoroughly thought-out and innovative fashion if it is to be possible to carry out connection and disconnection and replacement of power cables in a sufficiently quick and safe manner in the demanding environment where the connection substation is located.